Thermionic amplifier circuits



March 28 1939. w 3 PERCNAL 2,151,795

THERMIONIC AMPLIFIER CIRCUITS Filed Sept. 14, 1937 lNVENTOk w. s.PERCIVAL I W I ATTORNEY Patented Mar. 28, 1939 PATENT OFFICE THERM'IONICANEPLIFIER CIRCUITS William Spencer Percival, London, England, assignorto Electric & Musical Industries Limited Application September 14, 1937,Serial No. 163,758 In Great Britain September 24, 1936 2 Claims. 01.178-44) This invention relates to coupling circuits in thermionicvalveamplifiers and is particularly concerned with coupling circuits intendedto pass wide bands of frequencies. In television systems it is necessaryfor coupling circuits to be capable of passing frequency bands of theorder of four megacycles in width.

In the specification of my co-pending patent application Ser. No.126,317, filed February 18, 1937, it was pointed out that the highestvalue of frequency pass-band multiplied by amplifier stage gain can beobtained by allowing the shunt capacities .due to the grid and platecapacities of valves employed in an amplifier circuit to act as theshunt capacities of the sections of a filter terminated at one end andwith an extra capacity equal to that of a half section at the other end.

Again, inthe invention set forth in the specification of my co-pendingpatent application Ser. No. 163,446, filed September 11, 1937, use ismade of the leakage inductance of a coil forming an element of acoupling circuit designed to pass a wide range of frequencies, togetherwith a condenser, to provide a tuned circuit forming a section of afilter. In a particular arrangement described in the specification ofthat application a condenser is connected to a tapping point in aninductance coil connected in shunt across the amplifier circuit, theleakage inductance of that coil forming the required inductance.

According to the present invention a coupling circuit for a thermionicvalve amplifier designed to pass a wide band of frequencies is providedin which separate tuned sections of the circuit include inductanceelements connected to tapping points in an inductance connected in shuntacross the amplifier circuit, the inductance elements being coupledtogether so that the effect of the mutual inductance between the coupledinductance elements tends to neutralize the effect of the leakageinductance arising from the connection of the inductance elements to thetapping points in the shunt inductance. Preferably the coupling betweenthe coupled inductance is adjusted to produce substantially completeneutralization of the said leakage inductance.

In order that the nature of the invention and the method of carrying thesame into effect may be more clearly understood, a circuit arrangementto which the invention may be applied, together with explanatorycircuits will now be described, with reference to the accompanyingdrawing, in which Fig. 1 shows a form of amplifier circuit forming aband-pass filter to which the invention may be applied and Figs. 2, 3and 4 are explanatory diagrams.

Fig. 1 of the drawing shows a simple band pass amplifier circuitcomprising valves l and 2. The input circuits (not shown) of valve l isconnected to the control grid in the usual way. The output circuit,connected to the anode, comprises condenser C1 and inductanceLi, andcoupling inductance L2 across-which the input circuit of valve 2 isconnected. This input circuit includes the shunt branch includinginductance L3, condenser C3 and resistance R1. A high tension supply isconnected to the anode of valve I over high frequency choke L4, asindicated by the sign at the end of the lead, the lead being shunted bya suitable decoupling condenser C5 to prevent interfering efiects beingset up through the supply. The plate-cathode capacity of valve I is alsoeffective in the output circuit as represented by the shunt condenser 04shown dotted, and the grid-cathode capacity of valve 2 is effective inthe input circuit of the valve as shown by shunt condenser C2, alsoshown dotted. 'Ihe cathode leads of both valves include resistances R2and R3 respectively with by-pass condensers Cs and 07 respectively, inaccordance with wellknown practice.

A simple circuit of this form sulfers from practical disadvantages whenthe frequency passband to be handled is several megacycles wide and whenthe shunt capacities are small the self capacities of the inductancecoils L1 and L3 seriously upset the response characteristic, and theircapacities to earth materially increase the effective shunt capacitiesof the filter.

In Fig. 2 of the drawing the filter arrangement of Fig. 1 is representedmore clearly, elements corresponding to those in Fig. 1 being indicatedby like reference letters. In Fig. 2 the condenser C1 is shown as tappeddown the coil L4 as in the arrangement described in the specification ofapplication Ser. No. 163,446 already referred to and the inductanceelements L1 and L3 are shown tapped down the inductance coil L2, thevalves I and 2 of Figure 1 and associated immaterial elements C5, C6,C7, R2 and R3 having been omitted. The tapping down allows the values ofinductances L1 and L3 to be reduced and even without the mutualinductance shown effects a certain improvement, since theself-capacities of the coils L1 and L3 are less serious owing to theirsmaller inductances, and the method is satisfactory if the leakageinductance of the coil L2 is small. However, at high frequencies highpermeability cores which would be necessary in order to reduce theleakage are not very suitable owing to losses and the variation ofpermeability with frequency.

In the absence of mutual inductance between the coils L1 and L3 thecircuit equivalent to Fig. 2 would be as shown in Fig. 3 where elementsU2 and L4 are equivalent to the elements L2 and L4 of Fig. 2 and inwhich inductance Z represents the leakage inductance of coils L2 and L3.This leakage inductance is undesirable. To overcome this drawback,according to the present invention, coils L1 and L3 are coupledtogether. This, as is well known, increases the effective inductance ofeach of the coils so that they can be made smaller than they otherwisewould be and thus have reduced self-capacity, and moreover, with thisarrangement an apparent negative inductance, M say, is effectivelyinserted in series with the leakage inductance produced due to theauto-transformer effect between coils L1, L2 and. L3 and represented bythe coil 1. The coupling between the coils L1 and L3 is represented bythe dotted line M in Fig. 2. By adjustment of this coupling the value ofM can be made equal to l which is thus neutralized and the resultantcircuit is that of Fig. 4 which is the required standard band-passfilter. In this figure, elements L1, L'z, Us and U4 are the equivalentto elements L1, L2, L3 and L4 of Fig. 1.

The plate capacity of valve l and the grid capacity of the valve 2 maybe approximately equal. This is not an essential requirement, however,because if, for example, the capacity at the unterminated end of thefilter is low, the series resonant circuit comprising elements L1 and C1can be tapped down the inductance L4 and a suitable change made in thevalue of the coil L1.

What I claim is:

1. A coupling network for connection between a pair of vacuum tubeamplifiers and adapted to pass a wide band. of frequencies, comprisingan inductance included in the plate circuit of one tube and a secondinductance included in the grid circuit of the succeeding tube, a seriestuned circuit connected between the first inductance and a tapping pointon the second inductance, a second series tuned circuit connectedbetween said tapping point on the second inductance and the lowpotential end thereof, said series tuned circuits being provided withinductance elements which are coupled together so that the efiect of themutual inductance therebetween tends to neutralize the efiect of theleakage inductance arising from the connection of said inductanceelements to the tapping point on said second inductance.

2. A coupling network for connection between a pair of vacuum tubeamplifiers and adapted to pass a wide band of frequencies of the orderof several megacycles, comprising an inductance included in the platecircuit of one tube and a second inductance included in the grid circuitof the succeeding tube, a series tuned circuit including a condenser anda coil connected between a tapping point on the first inductance and atapping point on the second inductance, a second series tuned circuitincluding a condenser, a coil and a resistance connected between saidtapping point on the second inductance and the low potential endthereof, the coils of said series tuned circuits being coupled togetherso that the effect of the mutual inductance therebetween tends toneutralize the effect of the leakage ini

